The present invention is related to the field of pulp production, more particularly the invention relates to the field of refining wood chips into pulp for paper manufacturing.
Two broad categories of pulp manufacturing techniques are known in the art. The first technique is known as the digestion process, wherein lignocellulose fiber containing material (wood chips) are treated with chemicals and heat in order to break down the structure of the wood chips and produce pulp suitable for use in the paper making process. A second technique for producing pulp, known as the mechanical pulping process, involves passing lignocellulose fiber containing material, such as wood chips, through an attrition device where the fibers of the wood chips are mechanically separated. Variations of the mechanical pulping process are also known and include the thermo-mechanical pulping process (“TMP”). In the TMP process, wood chips are fed into a pressurized pre-heater, treated with steam and are subsequently ground into pulp. U.S. patent application Ser. No. 08/736,366, filed Oct. 23, 1996, “Low-Resident, High-Temperature, High Speed Chip Refining”, (now U.S. Pat. No. 5,776,305) discloses a further variation on the ground wood pulp process, whereby the wood chips are held at a temperature greater than the glass transition temperature (Tg) of the lignin in the wood chips for a period of time preferably less than 30 seconds, then immediately refined in a high speed disc refiner. According to the application, the wood chips are preferably subjected to a preheat environment of saturated steam at an elevated pressure in the range of 75-95 psi. (All values of pressure expressed as psi throughout this Specification including claims, refer to pounds per square inch gage pressure, i.e., psig). The assignee of the 08/736,368 application identifies the system and associated process as the “RTS”.
In both the chemical digestion and mechanical pulping techniques of making pulp, pulp wood logs are fed to chipper machinery where the logs are cut and sheared into pieces appropriately sized for subsequent processing. Once in chip form, the material is fed to a digestion reactor vessel, mechanical refining apparatus, or the pre-heating stage of the mechanical refining apparatus.